Tube waves, also called pressure waves or Stoneley waves, are plane pressure waves that propagate through tubular media, including annuli. These waves reflect from changes in the characteristic impedance of the medium. For example, the analysis of tube waves and/or their echoes can detect a fracture, bottom irregularity, or other features in a well. Several references describe the analysis of pressure wave reflections, such as US 2011/0267922, US 2012/0018150, U.S. 61/923,216, and U.S. Pat. No. 7,819,188.
A particular challenge in applying these techniques is a repeatable and reliable way to generate useful tube wave forms, e.g., with the desired rate of pressure change, frequency spectrum, peak power, total energy, repeatability, reliability, etc.
Pressure pulses are generated from water shock devices that rely on cavitation, e.g., Laake. A. et al., “A Generator for High Pressure Pulses in Liquids”, Journal of Sound and Vibration, 131(2), pp. 295-304 (1989). These devices require an impedance transformer, and only work at low pressure to produce cavitation bubbles that collapse.
Also, explosives are used to implode glass spheres for ocean seismic work, as described in Raymond, S. “Hollow Glass Spheres under Pressure in the Ocean—Experiments Show Interesting Properties,” IEEE Conf on Eng in the Ocean Environ and Mar Technol Soc, 11th Annu Meet, Proc; San Diego, Calif., USA, pp. 537-544 (22-25 Sep. 1975). Marine systems in general, and specifically such a glass sphere implosion, are not generally applicable to tube wave systems.
Various gas and water guns are available, including seismic gas guns miniaturized to fit into a wellbore as well as explosive guns firing blank shells, e.g., gas and water guns available under the trade designations ECHOMETER, NELGAR, LEUTERT, and so on. However, these guns are generally designed for use in compressible media, and suffer from the disadvantages of exhausting gas into the well in the case of gas guns, or require cavitation of the fluid for proper operation in the case of water guns.
Pressure pulses are also generated by opening and closing valves in a recirculating loop or in marine applications, e.g., U.S. Pat. Nos. 3,254,524, 3,376,949, but these are not designed for wellbore application. Similarly, negative pressure pulses are generated to locate cement wiper plugs by momentarily opening a vent valve, as in U.S. Pat. No. 6,401,814, but have the disadvantage that releasing fluid generates a waste stream. On the other hand, introducing exogenous fluid at a higher-than-system pressure into the system to generate a positive pressure pulse would require a fluid stream that is compatible with the pump and the system fluid, and would also entail a risk that the system can be over-pressured by the pump. One key wellsite hazard in pulse generation is the possibility that the wellhead may be closed while the pulse generator is operating.
The industry has an ongoing need for the development or improvement of tube wave generation methods and systems.